1. Field of the Invention
The present invention relates to an optical recording medium and, more particularly, to an optical recording medium which is adapted to record and reproduce information by application of a laser beam, wherein a constituent film thereof serving for light reflection has a smoothed surface for a higher recording density.
2. Description of the Related Art
To increase the recording density of an optical disk, the size of recording marks is minimized, for example, by employing an objective lens having a higher numerical aperture (NA).
The optical disk generally has a laminate structure including a substrate, a recording layer, a reflective film and a protective film. Where an optical lens having a high NA is employed to increase the recording density, the recording and reproducing accuracy tends to be reduced due to an increased aberration caused by variations in the thickness of the substrate and the tilt of the substrate. One conceivable approach to this problem is to cause a laser beam to be incident from the side of the protective film for the recording and reproduction. This recording and reproducing type is called “first surface recording (FSR) type” or “front illumination type”.
A commercially available conventional optical disk includes an optical disk substrate 1, and a lower dielectric film 2, a recording film 3, an upper dielectric film 4, a reflective film 5 and a protective film 6 stacked in this order on the substrate 1 as shown in FIG. 4. A light beam is incident from the side of the optical disk substrate 1.
Where the recording film is an exchange-coupled magnetic super-resolution medium of a three-layer structure (U.S. Pat. No. 5,623,458, U.S. Pat. No. 5,723,227), the lower dielectric film 2, a reproducing layer, an intermediate layer and a recording layer of the recording film 3, the upper dielectric film 4, the reflective film 5 and the protective film 6 are stacked in this order on the optical disk substrate 1.
On the other hand, an optical disk of the FSR type includes an optical disk substrate 1, and a reflective film 5, a lower dielectric film 2, a recording film 3, an upper dielectric film 4 and a protective film 6 stacked in this order on the substrate 1 as shown in FIG. 1. A light beam is incident from the side of the protective film 6.
Where the recording film 3 is an exchange-coupled magnetic super-resolution medium of a three-layer structure, the reflective film 5, the lower dielectric film 2, a recording layer 33, an intermediate layer 32, a reproducing layer 31, the upper dielectric film 4 and the protective film 6 are stacked in this order on the optical disk substrate 1 as shown in FIG. 2. The recording layer 33, the intermediate layer 32 and the reproducing layer 31 constitute the recording film 3.
The optical disk of the FSR type ensures a higher recording density. However, the recording film of the optical disk tends to have a rough surface as compared with the conventional optical disk, because the surface of the recording film is liable to be influenced by the surface state of the reflective film underlying the recording film thereby to have greater undulations thereon.
The surface roughness increases random reflection of the light to increase noise, resulting in reduction in CNR (carrier-to-noise ratio) and SNR (signal-to-noise ratio).
Particularly in a high NA optical system, the CNR and the SNR are remarkably reduced with a remarkable noise increase, though the amplitude of a reproduction signal is not changed. This increases reproduction errors.
Japanese Unexamined Patent Publication No. 2000-306271 proposes a method for producing an optical recording medium, which ensures reduction in disk noise. In this method, a reflective film 5 is formed on a surface of an optical disk substrate and subjected to a surface smoothing process by sputter-etching, and a lower dielectric film 2 and a recording film 3 are formed on the reflective film 5.
Where the recording film is thin on the order of 10 nm, an incident light beam passes through the recording film and is reflected on the reflective film.
Where the recording film 3 of the optical disk has a thickness greater than about 40 nm, however, the laser beam does not pass through the recording film. Accordingly, the laser beam is hardly reflected on the reflective film 5, but mostly reflected on the surface of the recording film 3.
Even if the reflective film 5 and the lower dielectric film 2 underlying the recording film 3 each have a smooth surface, the recording film 3 having a rough surface randomly disturbs the polarization plane of the light beam reflected on the surface thereof, so that greater noise occurs in detected light.
In the case of the magnetic super-resolution medium, the recording film 3 of the three-layer structure is stacked on the lower dielectric film 2. Even if the reflective film and the dielectric film underlying the recording film 3 each have a relatively smooth surface, the surface roughness is increased as the films are stacked one on another. As a result, the reproducing layer on which the light beam is reflected has a greater surface roughness than the underlying films.
The noise of interest is attributable to the surface roughness of a light reflection surface, so that the surface roughness of the reflection surface needs to be reduced.
In the case of the conventional optical disk medium shown in FIG. 4, a reflection surface 7 is defined between the lower dielectric film 2 and the recording film 3. Therefore, the noise can be reduced by smoothing the surface of the lower dielectric film 2. In the case of the optical disk of the FSR type, on the contrary, a reflection surface 7 is defined between the recording film 3 and the upper dielectric film 4. Even if the underlying lower dielectric film 2 is smoothed, the noise reduction cannot be achieved without smoothing the surface of the recording film 3 (or the reproduction layer 31) which serves as the reflection surface.